1. Field of the Invention
The present invention relates to a method of determining defects in photomasks for use in the manufacture of LSIs, which determines whether defects, if any, in a photomask may render the LISs defective.
2. Description of the Related Art
Photomasks for use in photolithography technology are getting more and more intricate, each having smaller and smaller elements arranged at a higher density, as LSIs have acquired higher integration density and become able to perform more functions in recent years. A photomask is a glass substrate on which a circuit pattern magnified four to five times has been printed by an electron-beam drawing apparatus. An exposure system projects reduced images of the circuit patterns to a silicon wafer, forming a circuit pattern on the silicon wafer. In recent years, LSIs have acquired higher integration density and become able to perform more functions. Now, as many as 50 to 70 photomasks are used to manufacture one LSI, in some cases. So many photomasks, which define the circuit pattern of an LSI, must be inspected, one by one, to see whether each satisfies the process precision demanded of it. Much labor and time are required to inspect photomasks, and the inspection cost is inevitably higher than the inspection cost of other products. Reportedly, the cost of inspecting photomasks amounts to about half the manufacturing cost of the photomasks.
An LSI is manufactured through a number of steps. First, its functions and logic are designed. Then, its circuit configuration is designed. Its circuit layout is designed. Based on the circuit layout, graphic data for forming photomask patterns (hereinafter also called design pattern data) are prepared. Using the design pattern data, photomasks are made. The patterns of the photomasks are transferred to a wafer by means of reduced projection exposure. Then, various processes are performed on the wafer, thereby manufacturing the LSI.
Most photomasks are prepared in the following method. First, in accordance with the design pattern data, an electron-beam exposure system or an excimer-wavelength photo exposure system applies a beam to the photosensitive resist coated on a light-shielding film that is provided on a photomask substrate (also called photomask blank). The photoresist is developed and etched, thereby providing a photomask. The photomask thus provided is inspected in a defect-detecting apparatus. The photomask, if found defective, is set in a defect-correcting apparatus that is designed to eliminate the defects.
A photomask is examined for defects by comparing the photomask with the design pattern data, or with a mask-drawing pattern data or an inspection pattern data prepared from the design pattern data, or by comparing the common patterns existing in the photomask with one another. Defects are detected at a defect-detection sensitivity set at the defect-detecting apparatus. The defect-detection sensitivity is defined by the size of the smallest defect the apparatus can detect. The defect-correcting apparatus performs a defect-removing process using a laser beam or an ion beam, or a defect-correcting process using focused-ion-beam assisted CVD, in order to illuminate the defects. The defects can hardly be perfectly corrected, however. When the photomask is inspected again, the defects may be detected again in some cases.
As mentioned above, the photomask is getting more and more intricate, each having smaller and smaller elements arranged at a higher density, as LSIs have acquired higher integration density and become able to perform more functions in recent years. In the case of a photomask complying with the 90-nm-to-45-nm rule applied to wafers, all defects detected by the defect-detecting apparatus cannot be corrected. Thus, it is difficult to provide photomasks that can pass the inspection that is performed at a prescribed defect-detection level. As pointed out above, the cost of inspecting photomasks amounts to a large part of the manufacturing cost of the photomasks. In view of this, it is demanded that the photomasks be inspected at as high an efficiency as possible.
JP-2002-258463-A, for example, discloses a method of inspecting photomasks, in which a defect-detecting apparatus compares the photomask pattern with the mask-drawing pattern data, thereby to detect defects, if any, in the photomask pattern. The mask-drawing pattern data includes graphic pattern data (i.e., drawing data) and graphic pattern data arrangement data (i.e., drawing-position data for the graphic pattern data), and has been subjected to optical proximity correction (OPC). From the mask-drawing data, intricate graphic patterns not OPC-corrected (e.g., test-element group (TEG) patterns) are previously extracted because these intricate graphic patterns are often detected as defective parts, though they should not be detected as such. Thus, in the process of inspecting the photomask pattern, the intricate graphic patterns are not detected as defective parts.
In the method disclosed in the above-identified publication, only the intricate graphic patterns (e.g., test-element group (TEG) patterns), which are often detected as detective parts in the process of inspecting the photomask pattern, though should not be detected as such, are previously extracted from the mask-drawing data that have been subjected to optical proximity correction (OPC). However, whether this or that graphic pattern should not be detected as a defective part is not necessarily based on the circuit pattern to be formed on the semiconductor substrate. Consequently, the criterion for selecting patterns that should not be detected as defective parts is not optimal.